Thin film interference filters are widely used in systems for optical measurement and analysis, such as Raman spectroscopy and fluorescence imaging, for example. Thin film interference filters, including optical edge and band pass filters, notch filters, and laser line filters (LLFs) are advantageously used in such systems to transmit light having specific wavelength bands and to reflect other light, including light that could otherwise constitute or generate spurious optical signals and swamp the signals to be detected and analyzed. Dichroic beam splitters utilize interference filter effects to reflect certain wavelengths or ranges of wavelengths and transmit other wavelengths or ranges of wavelengths. Failure or poor performance of such filters compromises the performance of systems in which they are used. Conventional design approaches for optical instruments that utilize thin-film filters are often constrained by inherent characteristics of these filters and long-standing practices for how these filters are designed and used.
As an example of one type of system that relies heavily on thin-film filters and benefits from high performance filter design, the simplified schematic diagram of FIG. 1 shows one type of imaging apparatus that is used for analysis of spectral characteristics of a sample. A fluorescence microscopy system 10 has a light source 12 with an illumination lens L2 that directs a beam of excitation energy, within a specific wavelength range, toward a sample 20 for analysis. Optical fluorescence occurs when absorption of light of the excitation wavelength(s) causes emission of light at one or more longer wavelengths. A succession of filters 22, 24, and a dichroic beam splitter 26 are used to isolate and direct the different wavelength bands of excitation and emitted light, respectively, to and from sample 20. The image-bearing emitted light from the sample is split into two components using a beam splitter 32 that is disposed in the path of this light at a 45-degree angle. Beam splitter 32 reflects a first wavelength band through a filter 38 and a lens L5 to a first detector 30a to form a first image. Beam splitter 32 also transmits a second wavelength band through a filter 36 and a lens L6 and to a second detector 30b to form a second image. Detectors 30a and 30b can be any of a number of light-sensing devices, such as a camera or charge-coupled device (CCD).
The surface flatness of dichroic beam splitters 26 and 32 affects a number of factors in the performance of fluorescence microscopy system 10. For light incident at high angles of incidence, such as the 45 degree angle of incidence (AOI) of a dichroic surface, the beam axis for transmitted light can be slightly laterally shifted relative to the axis of incoming light to the surface. Furthermore, if both surfaces of the dichroic are appreciably curved, such that the dichroic has the shape of a bent parallel plate, the beam axis for transmitted light can be slightly diverted and therefore non-parallel to the axis of incoming light to the surface. Light reflected from the tilted surface, however, presents even more of a problem. For example, unless the reflective surface is flat to within close tolerances, the focus of the excitation beam from light source 12 can be shifted along the axis away from the focal plane of the focusing lens and the size of the focused point can be compromised. Similarly, the focal plane of the emitted light that is reflected by beam splitter 32 can be shifted along the axis of light away from detector 30a and the image can be distorted.
There can be additional problems related to flatness with specific types of microscopy systems as well. For example, for a type of laser based fluorescence microscopy termed Total Internal Reflection Fluorescence or TIRF microscopy, the laser beam needs to be focused at the back focal plane of the objective lens L1. In yet another fluorescence microscopy technique, termed Structured Illumination microscopy, there is a grid pattern in the light path between lens L2 and filter 22. This grid pattern needs to be imaged onto sample 20, as shown in FIG. 1. For both TIRF and Structured Illumination microscopy, the relative flatness of the dichroic surface that corresponds to beam splitter 26 affects how well the measurement apparatus performs. Similarly, if there is unwanted curvature of dichroic beam splitter 32, one or both of these problems can easily occur: (1) the position of the focal plane shifts and (2) the size or shape of the focused spot changes. Either of these two effects, or their combined effect, can significantly compromise the image quality of detector 30a. Aberrations resulting from this focal shift and degradation may not be easy to correct and can adversely affect the overall imaging performance of the microscopy system.
Imperfect flatness can be a particular problem when using dichroic beam splitters with laser light, whether in microscopy or in other applications. For this reason, dichroic surfaces rated for use with lasers must meet higher standards for flatness and are more costly than dichroic surfaces that are used for other light sources.
Dichroic coatings are typically formed by thin film deposition techniques such as ion beam sputtering. These fabrication methods require deposition onto a flat substrate, but tend to add significant amounts of mechanical stress as they are applied. This stress, if not corrected in some way, can cause some amount of bending or warping of the underlying substrate, frustrating attempts to maintain suitable flatness. For this reason, many types of commercially available dichroic beam splitters encase the dichroic coating within a prism. Encasement solutions, however, present other problems, including practical difficulties in alignment, the need for optical adhesives that can both withstand the optical and temperature environment while closely approximating the refractive index of the surrounding glass, and other shortcomings. The dichroic coating works best when it is disposed directly in the path of incident light; encasing the coating within glass or other substrate introduces optical problems, such as absorption and scattering, that can degrade optical performance.
Another inherent shortcoming of conventional thin-film deposition techniques used for forming a dichroic coating relates to limitations of the needed surface flatness for accurate deposition. It can be very challenging, and in many cases, can be impractical to apply a dichroic coating to a surface having any appreciable curvature. This constrains dichroic coatings from being readily applied to lenses, mirrors, and other non-plano surfaces.
Thus, there would be advantages to methods that would allow additional control of surface flatness as well as allowing conformance of dichroic filters to non-plano surfaces.